Refineries, chemical plants, and other manufacturing facilities often need or are required by law to monitor emissions. Airborne or vapor emissions may be monitored, for example, to avoid or detect atmospheric concentrations of chemicals that might create: unhealthy or illegal emissions to the environment outside the facilities; unhealthy or illegal chemical exposures of personnel; or risks of fire or explosions. In particular, industrial facilities that handle petroleum commonly employ detectors that measure Total Organic Vapor (TOV) or Total Petroleum Hydrocarbon (TPH) to ensure compliance with environmental regulations and for personnel and asset safety. Several types of chemical detectors are currently in use to measure TOV or TPH.
Flame ionization detectors (FIDs) are common for measurement of TOV or TPH. FIDs typically use a hydrogen flame to ionize organic vapors in the air or sample gas passing by the flame, and an electrical measurement of the resulting ions can then indicate the concentration of organic vapors exposed to the flame. FIDs can detect and measure a broad range of hydrocarbons from the lightest, i.e., methane or C1, up to the heaviest hydrocarbons that may be of interest, e.g., C11. FIDs are thus good detectors of TOV and TPH, but FIDs also have drawbacks. In particular, FIDs are generally more sensitive to aliphatic (or chained) hydrocarbons because aliphatic hydrocarbons burn more efficiently than do aromatic (or ringed) hydrocarbons. Also, FIDs may be inconvenient to use because FIDs require a supply of hydrogen, e.g., a hydrogen generator or regularly refilled or replaced hydrogen canisters. The flame in a FID may also create risks.
Photoionization detectors (PIDs) use an ultraviolet lamp to ionize organic vapors so that ionized organic compounds can be electrically measured. PIDs do not require a hydrogen supply or a flame and can be simply operated using portable or readily available electrical power. PID may also be more accurate than FIDs and some other types of detectors and may provide measurements with accuracies in the parts-per-million (ppm) range. However, PIDs may be most sensitive to aromatic hydrocarbons (e.g., BTEX compounds, benzene, toluene, ethylbenzene, and xylenes), which have lower ionization energies than to some lighter aliphatic hydrocarbons, but PIDs can also efficiently detect heavier aliphatic hydrocarbons, particularly if the PIDs employ UV lamps producing photons with shorter wavelength and therefore higher photon energies. PIDs are thus excellent at detecting heavier hydrocarbons that present the greatest health risks. However, light aliphatic hydrocarbons such as methane (CH4), ethane (C2H6), or propane (C3H8), which are sometimes referred to herein as C1, C2, or C3, have ionization energies that are higher than the photon energies of UV lamps commonly employed in PIDs, making many PIDs inefficient at detecting lighter hydrocarbons. As a result, current PIDs may not provide accurate TOV or TPH measurements. Also, PID measurements of volatile organic compounds (VOCs) are known to be sensitive to moisture and methane because water and methane molecules can partially absorb the UV light from the UV lamp and cause a VOC reading to drop when there is a high level of moisture or methane in the gas samples. (See, for example, U.S. Pat. No. 4,778,998.)
Current infrared (IR) detectors are effectively spectrophotometers that can measure hydrocarbon concentrations by measuring absorption of IR radiation at specific wavelengths, typically wavelengths between about 3.3 and 3.5 microns, characteristic of the hydrogen-carbon bonds of petroleum hydrocarbons. IR detectors can be used to measure hydrocarbons but may not uniformly detect all hydrocarbons. In particular, lighter hydrocarbons may have absorption peaks in the range of an IR detector, but the peaks in the absorption spectra of some heavier hydrocarbons may lie at the edge of or outside the detection range of an IR detector.
Some other systems for measuring organic vapors that have been considered include pellistors, catalytic hydrocarbon detectors, detector tubes, fiber optic chemical sensors, colorimetric test kits, turbidimetric test kits, and immunoassay test kits.
The drawings illustrate examples for the purpose of explanation and are not of the invention itself. Use of the same reference symbols in different figures indicates similar or identical items.